Process for electrocoating an electrically conductive substrate

ABSTRACT

A process for coating an electrically conductive substrate, connected as the cathode, with a cationic coating agent from an electrocoating bath and subsequently hardening the coating, is characterized in that an aqueous dispersion made from 
     A. cationic, synthetic resins which have been protonated with acids, contain basic groups and are in a dissolved or dispersed form and 
     B. dispersed, finely divided, ionic plastics 
     is used as the cationic coating agent.

The invention relates to a process for coating an electricallyconductive substrate, connected as the cathode, with a cationic coatingagent from an electrocoating bath and subsequently hardening thecoating. The invention also relates to the coating agent for carryingout the coating process.

West German Published Application No. 2,248,836 discloses a process inwhich coatings are deposited onto the cathode in an electrocoating bath,which contains a cationic resin together with a pulverulent, nonionic,synthetic resin. One of the disadvantages of the known electrocoatingbaths is that they are unstable and segregate by precipitating thepulverulent contents. If, for example, stirring equipment fails, theseelectrocoating baths are frequently almost impossible to re-stir lateron.

A further disadvantage of the known coating agents is that unevencoatings are obtained on differently pretreated substrates. Remote partsare also coated less well. Different film thicknesses are obtained onsubstrates with horizontal and vertical areas.

It is therefore the object of the invention to provide improvedelectrocoating baths, avoiding these disadvantages.

This object is surprisingly achieved by a process for coating anelectrically conductive substrate, connected as the cathode, with acationic coating agent from an electrocoating bath and subsequentlyhardening the coating, wherein an aqueous dispersion made from P1 A.cationic, synthetic resins which have been protonated with acids,contain basic groups and are in a dissolved or dispersed form and

B. dispersed, finely divided, ionic plastics is used as the cationiccoating agent.

Aqueous dispersions which are preferentially suitable are thosecontaining pigments and/or fillers and/or those water-miscible organicsolvents which do not either incipiently dissolve or incipiently swellthe finely divided plastics.

In a particularly preferred embodiment of the invention, the finelydivided ionic plastics can themselves contain pigments and/or fillers.

Not only the finely divided, ionic plastics but also the cationicsynthetic resins can be cathodically deposited evenly from the aqueousdispersions according to the invention, and they produce, after a briefcoating time, coatings of up to 150 μm which have, after stoving,outstanding mechanical properties, such as high hardness and scratchresistance together with good elasticity and firm adhesion to thesubstrate.

After stoving at temperatures of up to 200° C., for a stoving time ofabout 15 minutes, the coatings have exceptionally good corrosionresistance. Values of up to 1,000 hours are achieved in the salt spraytest as specified in German Industrial Standard DIN 50,021.

It has also been found that the surface of the stoved coating is sosmooth that a single top layer of lacquer is sufficient to achieve alacquer coating with a good appearance.

The coating agent is distinguished by a very good bath stability andlong shelf life.

The aqueous dispersions used in the process according to the inventionmake it possible for any electrically conductive metallic workpieces,preferably workpieces from ferrous metals, to be coated. The workpiecesto be coated are immersed in an electrocoating bath and connected as thecathode.

The cationic synthetic resin which contains basic groups is present inthe aqueous dispersion in a protonated form and serves as a carrierresin for the component B. It will be designated "carrier resin" in thetext which follows. The carrier resin is protonated with suitableinorganic and/or organic acids, preferably water-soluble carboxylicacids, and, in the protonated form, it is soluble or dispersible inwater or can be mixed and diluted with water. The pH value of theaqueous dispersion can be adjusted to a value between 1 and 9.

Suitable acids are virtually all known inorganic and organic acids, suchas, for example, hydrochloric acid, sulfuric acid, phosphoric acid,carbonic acid, p-toluenesulfonic acid, acetic acid, propionic acid,formic acid, citric acid, lactic acid, malic acid, fumaric acid, maleicacid and phthalic acid and also the half-esters of fumaric acid, maleicacid and phthalic acid with monohydric or polyhydric aliphatic alcohols,such as methanol, ethanol, propanol or ethylene glycol. The best resultsare obtained using acetic acid, lactic acid and formic acid, which aretherefore proposed as preferentially suitable protonating agents.

The carrier resin is preferably used in coating agents for the cathodicelectrocoating lacquering of electrically conductive substrates, forexample metal parts made of aluminum, brass, copper, iron, steel andalloys of iron with other metals, which may have been pre-treatedchemically, for example phosphatized.

The aqueous dispersion used in the process according to the inventioncontains as component A cationic synthetic resins containing basicgroups. Products from the reaction of a resin which contains epoxidegroups with primary and/or secondary amines or products from thereaction of a resin which contains epoxide groups with Mannich baseswhich are free from epoxide groups are preferentially suitable. Thissynthetic resin, together with the component B, forms an aqueousdispersion stable at room temperature.

Any monomeric or polymeric compound or a mixture of such compounds whichon average contains one or more epoxide groups per molecule can be usedas the resin which contains epoxide groups. Polyepoxide compounds having2 to 3 epoxide groups per molecule are preferred. Polyglycidyl ethers ofpolyphenols, such as of bisphenol A, are a particularly highly suitableclass of polyepoxides. These ethers are obtained, for example, byetherifying a polyphenol by means of epichlorohydrin or dichlorohydrinin the presence of alkali. Examples of a phenolic compound for thesepolyepoxides are bis-(4-hydroxyphenyl)-2,2-propane,4,4'-dihydroxybenzophenol, bis-(4-hydroxypehnyl)-1,1-ethane,bis-(4-hydroxyphenyl)-1,1-isobutane,bis-(4-hydroxy-tert.-butylphenyl)-2,2-propane,bis-(2-hydroxynaphthyl)-methane and 1,5-dihydroxynaphthalene. In manycases it is advantageous to use polyepoxides which have somewhat highermolecular weights and aromatic groups. They are obtained by reacting thediglycidyl ether with a polyphenol, such as bisphenol A, and thereafterreacting this product further with epichlorohydrin to give apolyglycidyl ether. The polyglycidyl ether obtained from polyphenolspreferably contains free hydroxyl groups in addition to the epoxidegroups.

The polyglycidyl ethers of polyphenols can be used as such, but it isfrequently advantageous to react some of the reactive positions(hydroxyl groups or in many cases epoxide groups) with a modifyingmaterial, which modifies the film properties of the resin. Well knowncandidates for such a reaction are an esterification using carboxylicacids, in particular fatty acids, and/or an etherification usingmonoalcohols or polyalcohols. Saturated fatty acids and in particularpelargonic acid are particularly suitable carboxylic acids.

Ethylene glycol, propylene glycol, butane-1,2-diol, hexane-1,4-diol,neopentyl glycol, dimethylolcyclohexane and perhydrobisphenol A, andalso polyesters, such as, for example, preferably linear polyestershaving terminal OH groups, are suitable polyalcohols. The epoxide resinscan also be modified with organic materials which contain isocyanategroups or with other reactive organic materials.

Another group of suitable polyepoxides is obtained from novolacs orsimilar polyphenol resins.

Polyglycidyl ethers of polyhydric alcohols are also suitable. Examplesof such polyhydric alcohols which may be mentioned are ethylene glycol,diethylene glycol, triethylene glycol, 1,2-propylene glycol,1,4-propylene glycol, pentane-1,5-diol, hexane-1,2,6-triol, glycerol andbis-(4-hydroxycyclohexyl)-2,2-propane. Polyglycidyl esters ofpolycarboxylic acids can also be used and are obtained by reactingepichlorohydrin or similar epoxide compounds with an aliphatic oraromatic polycarboxylic acid, such as oxalic acid, succinic acid,glutaric acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid anddimerized linoleic acid. Glycidyl adipate and glycidyl phthalate areexamples.

Copolymers formed from a reaction of unsaturated compounds which containan epoxide group, for example glycidyl methacrylate, glycidyl acrylate,N-glycidylacrylamide, allyl glycidyl ether and N-glycidylmethacrylamide,with another unsaturated monomer which can be copolymerized with theformer are also suitable.

A reaction product is formed on reacting materials which contain epoxidegroups with an amine. The amine used can be primary or secondary,secondary amines being particularly highly suitable.

Examples of such amines are monoalkylamines and dialkylamines, such asmethylamine, ethylamine, propylamine, butylamine, dibutylamine,dimethylamine, diethylamine, dipropylamine, diisopropylamine,methylbutylamine, dibutylamine, diethanolamine, diamylamine,diisopropanolamine, ethylaminoethanol, ethylaminoisopropanol,ethanolamine, ethylenediamine, diethylenetriamine, methylcyclohexylamineand dicyclohexylamine.

Low-molecular amines are used in most cases, but it is also possible toemploy high-molecular monoamines, in particular when it is intended toincrease the flexibility of the resin by incorporating such amines.Similarly, mixtures of lower-molecular and higher-molecular amines canalso be used for modifying the properties of the resin.

The amines can also contain other groups, but these must not interferewith the reaction of the amine with the epoxide group and must also notlead to a gelling of the reaction mixture.

The reaction of the amine with the compound which contains epoxidegroups frequently occurs even during mixing of these materials. However,heating the reaction mixture at moderately elevated temperatures may bedesirable, for example at 50° to 150° C., but reactions can also becarried out at lower and higher temperatures. To terminate a reaction itis frequently advantageous to increase the temperature, at least alittle, towards the end of the reaction for a sufficient period toensure complete conversion.

An amount of amine which is sufficiently large to ensure that, after theresin has been protonated by the addition of an acid, it takes on asufficiently cationic character to permit the dilution with water shouldbe used for the reaction with the compound containing epoxide groups.Essentially all epoxide groups of the resin can be reacted with anamine. However, it is also possible to leave excess epoxide groups inthe resin, which hydrolyze with the formation of hydroxyl groups oncontact with water.

The whole structure of the cationic synthetic resin which contains aminogroups makes it possible to ensure that, after it has been protonatedwith acids as a carrier resin, finely dividided ionic plastics can bedispersed in it in such a way that stable aqueous dispersions areproduced, even at a pH value of over 7, from which the cathodicdeposition of the coatings can be carried out at these pH values ofbetween 7 and 9.

The resins which contain epoxide groups can also be reacted with Mannichbases which are free from epoxide groups, instead of with the amines, inorder to form the cationic synthetic resins. This reaction is known andhas been described ih the West German Published patent application Nos.2,419,179, 2,320,301, 2,357,075, 2,541,801, 2,554,080 and 2,751,499.

Those epoxide-free Mannich bases which are preferentially suitable arethose obtained by reacting the components

(a₁) condensed phenols which are free from ether groups and contain atleast two aromatic rings and at least two phenolic hydroxyl groups,and/or

(a₂) condensed phenols which contain ether groups and at least twoaromatic rings and at least one phenolic hydroxyl group,

(a₃) secondary amines having at least one hydroxyalkyl group, ifappropriate mixed with

(a₄) secondary dialkylamines or dialkoxyalkylamines which do not containfree hydroxyl groups, and

(a₅) formaldehyde or compounds which split off formaldehyde.

The following may be said about the individual components:

Condensed phenols which are free from ether groups and contain at leasttwo aromatic rings and at least two phenolic hydroxyl groups, which areparticularly suitable as component (a₁), are condensed phenols of thegeneral formula ##STR1## wherein the hydroxyl groups are in theortho-position or para-position in relation to X, and X is astraightchain or branched, divalent aliphatic readical having 1 to 3carbon atoms, or SO₂, SO or ##STR2## (in which R denotes an alkylradical having 1 to 6 C atoms); bisphenol A is particularly suitable.Low-molecular reaction products formed from phenols and formaldehyde,so-called novolacs, can also be employed.

If appropriate, it is possible to use, as a mixture with the condensedphenols (a₁) or instead of the latter, further condensed phenols (a₂)which contain at least one phenolic hydroxyl group and, in addition,also one or more ether groups in the molecule. These products have thegeneral formula

    HO--B--[O--E--O].sub.n --H

or

    HO--B--[O--E--O].sub.n --P

wherein B represents the radical ##STR3## and X has the meaningindicated above, E represents a radical which contains hydroxyl groupsand has been obtained by adding an epoxide compound onto a phenolichydroxyl group, P represents a phenyl or alkylphenyl radical and nrepresents an integer from 1 to 3, and wherein epoxide resins, such as,for example, diglycidyl ethers of bisphenol A, pentaerythritol,glycerol, trimethylolpropane, glycol, glycol ethers and otherpolyhydric, preferably dihydric to tetrahydric alcohols, are preferablyemployed as the epoxide compounds (for E).

If the condensed phenols (a₂) are to be used on their own, it isadvantageous to select those based on triglycidyl or tetraglycidylethers.

Other suitable compounds containing epoxide groups arenitrogen-containing diepoxides, such as are described in U.S. Pat. No.3,365,471, epoxide resins obtained from 1,1-methylene-bis-(5-substitutedhydantoin) in accordance with U.S. Pat. No. 3,391,097, diepoxidesobtained from bisimides in accordance with U.S. Pat. No. 3,450,711,epoxylated aminomethyldiphenyl oxides according to U.S. Pat. No.3,312,664, heterocyclic N,N'-diglycidyl compounds according to U.S. Pat.No. 3,503,979, aminoepoxy phosphates according to British Pat. No.1,172,916 or 1,3,5-triglycidyl isocyanurates.

Components (a₂) which are particularly preferred are the products formedfrom the reaction of diglycidyl ethers of bisphenol A or of polyhydricaliphatic alcohols, such as pentaerythritol, trimethylolpropane andglycerol, with bisphenol A and, if appropriate, phenol, which containphenol groups and are virtually free from epoxide groups. Such productsgenerally have molecular weights from 650 to 1,300 and epoxide valuesfrom 0.004 to 0.01 and can be prepared, for example, at temperaturesbetween 160° and 180° C., or at correspondingly lower temperatures inthe presence of catalysts for the reaction.

The condensed phenols (a₂) contain aliphatically bonded hydroxyl groups.Some of these are formed from the epoxide groups of the epoxide resins(E) in the reaction of the latter with the bisphenols (B) or with thephenols (P). However, hydroxyl groups can also be present already in theepoxide resins themselves, if the latter have been prepared by reactingalcohols of a functionality higher than dihydric (for examplepentaerythritol, trimethylolpropane or glycerol) with 2 moles ofepichlorohydrin.

In the case which is in itself preferred, in which mixtures of thecomponents (a₁) (a₂) are employed, the ratio by weight of the twocomponents is between 1:0.1 and 1:5.

Examples of suitable secondary amines (a₃) which contain at least onehydroxyalkyl group are alkylethanolamines or alkylisopropanolamineshaving 1 to 6 carbon atoms in the alkyl group. Dialkanolamines ofalcohols having 2 to 6 carbon atoms, in particular diethanolamine, andalso mixtures of these dialkanolamines with alkylalkanolamines arepreferred, however.

The secondary amines (a₃) which are incorporated in the Mannich bases asdialkanolaminomethyl groups and alkylalkanolaminomethyl groups are ofconsiderable importance for the degree of dispersibility of the bindersin the desired pH range of 6.0 to 10.2 and for the crosslinking of thesystem.

Suitable secondary dialkylamines or dialkoxyalkylamines (a₄), which areemployed conjointly with the amine (a₃) containing hydroxyalkyl groupsin the preparation of the Mannich bases, are those of the generalformula ##STR4## in which R₁ and R₂ are identical or different andrepresent a straight-chain or branched aliphatic radical which has 2 to10 carbon atoms and may contain alkoxy groups. Examples of suitablesecondary amines of this type are di-n-butylamine, di-n-propylamine,diisopropylamine, di-n-pentylamine, di-n-hexylamine, di-n-octylamine,di-2-ethylhexylamine and di-2-alkoxyethylamines, such as, for example,di-2-methoxyethylamine, di-2-ethoxyethylamine or di-2-butoxyethylamine,and also secondary amines in which R₁ and R₂ are linked to form a ring,such as, for example morpholine or piperidine.

Di-n-butylamine, di-2-ethylhexylamine and di-n-hexylamine arepreferentially suitable. The mode of action of these secondary amines(a₄) consists chiefly in influencing the stability properties of thebinders, and in addition they contribute to the leveling and to the"internal plasticization" of the lacquer films produced from thebinders. They also make a certain contribution to the crosslinking.

As a result of their mode of preparation, the secondary amines can alsocontain, inter alia, corresponding primary amines, but their content ofthese should not exceed 20 percent by weight of the secondary amine. Theratio by weight of the components (a₃) and (a₄) can be between 1:10 and1:0.1, preferably between 1:2 and 2:1.

Aqueous or alcoholic, such as, for example, butanolic, solutions offormaldehyde or paraformaldehyde or mixtures thereof are used asformaldehyde or compounds which provide formaldehyde (a₅).

The Mannich bases are prepared by the customary methods indicated in theliterature (see, for example, Houben-Weyl, Methoden der organischenChemie (Methods of Organic Chemistry), volume XI/1, page 731 (1957)),preferably by carrying out the reaction at temperatures between 20° and80° C. The proportions of the starting materials employed depend on theparticular properties desired, the molar ratio of the components (a₁)and (a₂) to the components (a₃) and (a₄) being preferably 1:0.75 to 1:3.In general, however, about one mole of secondary amine is employed foreach phenolic hydroxyl group. The quantity of (a₅) is at least one mole,relative to one mole of secondary amine.

The Mannich bases which are free from epoxide groups are reacted in aquantity of 50 to 90, preferably 60 to 80, percent by weight, with 5 to50, preferably 10 to 30, percent by weight of the epoxide resin. Thereaction of the components is carried out in general at temperatures of20° to 100° C., preferably 60° to 80° C., if appropriate in the presenceof organic solvents, such as, for example, alcohols, glycol ethers andketones. The reaction product obtained is substantially free fromepoxide groups.

Some of the aliphatically bonded hydroxyl groups of the component A can,if appropriate, be converted into urethane groups. In the reaction ofthe hydroxyl groups with partially blocked polyisocyanates, resins freefrom epoxide groups are used preferably. If epoxide resins based onpolyhydric aliphatic alcohols, for example pentaerythritol, are used,the attack of the isocyanate takes place preferentially at the freeprimary alcohol group; there is only a secondary reaction at thesecondary alcohol group which has been formed from the epoxide ring.

Any amino or imino groups which may be present can also react with thepartially blocked polyisocyanates, which can be desirable in many cases.

The reaction is usually carried out at temperatures from 50° to 120° C.,preferably from 70° to 100° C., and conventional catalysts for theformation of polyurethanes, such as, for example, dibutyltin dilaurate,can be present. The reaction is carried out in the absence of polarsolvents; it is preferable to carry out the reaction in the melt, butinert diluents can also be present.

Aromatic diisocyanates, such as toluylene diisocyanates or xylylenediisocyanates or dimers and trimers thereof, are particularly suitableas partially blocked polyisocyanates. However, it is also possible touse aliphatic diisocyanates, such as hexamethylene diisocyanate, andalso the prepolymers which are prepared by reacting polyols or polyetherpolyols with an excess of polyisocyanates. Preferential blocking agentsare aliphatic alcohols, which can have a straight-chain, branched orcyclic structure, such as, for example, methanol, ethanol, n-, iso- ortert.-butanol, hexanol, ethylhexanol, furfuryl alcohol, cyclohexanol,alkylglycols, alkyldiglycols and alkyltriglycols. Other known blockingagents such as oximes, lactams, ketones or malonic esters can, however,also be used.

It is possible, without difficulty, to modify only a fraction of theMannich bases or of the epoxide resins with polyisocyanates, whetherthis is because epoxide compounds containing or not containing aliphatichydroxyl groups are present alongside one another or whether further,unmodified epoxide compounds are added after the reaction with apolyisocyanate has been carried out.

The proportions in the reactions with the partially blockedpolyisocyanates are preferably so chosen that there is 0.01 to 1.0,preferably 0.05 to 0.5, mole of urethane groups to one mole of basicnitrogen in the finished reaction product, counting both the urethanebond between reaction product and polyisocyanate and the urethane bondbetween blocking agent and polyisocyanate.

The whole structure of the reaction product makes it possible to ensurethat, after it has been protonated with acids as a carrier resin, finelydivided, ionic plastics can be dispersed in it in such a way thatstable, aqueous dispersions are formed, from which the cathodicdeposition of the coatings can be carried out. In its protonated formthe carrier resin can be diluted with water. If required, it is possiblefor additional solvents in amounts up to 5%, relative to the solidcarrier resin, also to be present, such as, for example, alcohols, suchas isopropanol, propanol, butanol, glycols or glycol ethers, such asethylene glycol, propylene glycol, ethylene glycol monoethyl ether,ethylene glycol monopropyl ether or ethylene glycol monobutyl ether, orother solvents, such as tetrahydrofuran, aliphatic and/or aromatichydrocarbons, esters, ethers or ether-esters, in order to affectadvantageously the dissolving properties and dispersing properties ofthe carrier resin.

It is an important characteristic of the invention that the aqueousdispersion contains finely divided ionic plastics dispersed in it as thecomponent B.

These plastic powders are solid and easy to grind at room temperature upto temperatures of 100° C. They are not reactive in the sense ofundergoing film formation to give high-molecular materials attemperatures as low as room temperature on their own or together withother compatible resins, such as the cationic carrier resin. However,under the conventional stoving conditions, which are between 140° C. and220° C., preferably above 160° C., they melt and combine with thecationic carrier resin on the coated substrate to form a compatiblefilm.

In respect of chemical structure, the same synthetic resins can be usedfor the component B as can be used for the component A, with the provisothat when used as the ionic plastics of the component B they must beprovided at room temperature in a solid and finely divided form. As arule, this is the case when they have a glass transition temperature of30°-150° C., preferably of 40°-130° C.

Plastic powders which are preferentially suitable for use as thecomponent B belong to the group comprising epoxide resins, polyesterresins, acrylate resins, polyurethane resins and polyamide resins. Theycontain ammonium, sulfonium or phosphonium groupings as ionic groups.Those finely divided plastics which contain ammonium structures areparticularly advantageous.

They are prepared by known methods, for example by reacting epoxideresins or modified epoxide resins with primary or secondary amines andsubsequently neutralizing the amino groups with acids. The resins whichcontain epoxide groups and were listed initially for the component A aresuitable as the epoxide resins or modified epoxide resins.

Those finely divided plastics which are obtained by reacting resinswhich contain epoxide groups with tertiary ammonium salts to givequaternary ammonium compounds are also suitable as component B. They arelikewise known and have been described, for example, in West Germanpublished application No. 2,531,960.

Pulverulent, finely divided acrylate resins having basic nitrogen groupsare also suitable as component B. Such acrylate resins are produced, forexample, by using, for example, dimethylaminoethyl acrylate,diethylaminoethyl acrylate, dimethylaminopropyl acrylate,diethylaminopropyl acrylate or the corresponding methacrylates ascomonomers in the polymerization and subsequently neutralizing withacids the copolymer obtained.

Ionic polyurethane powders, which are prepared by using as a polyolcomponent, for example triethanolamine or tripropanolamine, are alsosuitable as component B.

All the powders of this type can be employed in the aqueous dispersionas component B, provided that they are compatible with the carrierresin. Incompatibility can readily be recognized by the fact that thecoating system separates into two layers when stoved.

The ionic plastic powder can be dispersed in this form in the aqueousdispersion. However, it is also possible to use a plastic powdercontaining fillers. In this case, the pigments and/or fillers havealready been incorporated during the preparation of the ionic plasticpowder. The aqueous dispersion itself can then be free from pigments.

The aqueous dispersion according to the invention is not limited merelyto containing a single ionic resin. Mixtures of two or more diffferentplastic powders can also be present. In this case, one or other plasticpowder can contain pigments and/or fillers, but the other plastic powdercan be free from these additives. The plastic powders employed can alsocontain, in addition to pigments and fillers, small quantities ofhardening agents and other additives which regulate the flow behavior ofthe powder during stoving. The action of these additives incorporatedinto the plastic powder cannot be adversely affected by the aqueousdispersion.

As is customary in other coating agents, the aqueous dispersion cansimilarly contain auxiliaries which can be deposited by electrophoresis,such as, for example, pigments, fillers, hardening catalysts, agents forimproving flow, anti-foaming agents and agents for improving adhesion.In many cases it is advantageous for the process according to theinvention if the aqueous dispersion also has an additional content ofcrosslinking agents, with which the components A and B react bycrosslinking at stoving temperatures. Examples of possibe crosslinkingagents are aminoplast resins, such as, for example,melamine-formaldehyde resins or urea-formaldehyde resins, phenoplastresins or fullyblocked isocyanates.

The ratio between the components A and B is important when using theaqueous dispersion for the production of stoved coatings on the surfaceof electrically conductive substrates, connected as the cathode, bycathodic deposition from a coating bath in a cathodic electrocoatinglacquering process, and the average particle size of the component B isalso important for the quality of the coating deposited.

The best results are obtained in the cathodic deposition if there are0.1 to 100 parts by weight of the component B, preferably 0.5 to 10parts by weight of the component B, to 1 part by weight of the componentA, relative to the pigment-free and filler-free powder.

The aqueous dispersion contains, in addition to the component A and thecomponent B, also 0 to 10 parts by weight of pigments and/or fillers,preferably 2 to 5 parts by weight.

The particle size of the component B is an important factor. It shouldhave a particle size distribution in which at least 95% of the particlesare smaller than 30 μm. The best results are obtained and hence thoseplastic powders are preferred in which at least 95% of the particles aresmaller than 10 μm. The particles of the plastic powder are more readilycoated by the carrier resin as their size decreases. For this reason thecathodic deposition of finer particle sizes is simpler and more even.

The aqueous dispersion is prepared by the methods which are known in thepaint industry. Thus, the plastic powder can be stirred directly intothe aqueous solution or dispersion of the protonated carrier resin bymeans of a high-speed dispersing apparatus. Another possibility lies injointly incorporating the plastic powder, together with the desiredpigments and/or fillers, into the aqueous solution of the protonatedcarrier resin in a ball mill or stirred ball mill.

A further possible means of preparing the aqueous dispersion is mixingan aqueous suspension of a plastic powder directly into the aqueoussolution of the carrier resin. This method dispenses with the expensivegrinding process by means of a sand mill or a stirred ball mill.

In order to facilitate the preparation of the aqueous dispersion, it ispossible to effect the incorporation of the solid component in thepresence of small quantities of emulsifiers. Examples of suitableemulsifiers are nonionic emulsifiers of the type of ethlene oxideadducts of varying chain lengths, such as, for example, alkylphenolsmodified with ethylene oxide, for example tertiary octylphenol which hasbeen modified with 5 to 40 ethylene oxide units, and also higheraliphatic alcohols modified with ethylene oxide, such as, for example,lauryl alcohols having 15 to 50 ethylene oxide units, and also similarlymodified longchain mercaptans, amines or fatty acids. Preferred mixtureshave at least two ethylene oxide adducts in which the ethylene oxideunits have different values. The bath stability and the properties ofthe coating are not substantially affected by the additives.

Cationic emulsifiers, such as, for example, low-molecular aminocompounds which contain OH groups and which have been protonated withorganic or inorganic acids, are also suitable. The quantities ofemulsifiers should not exceed 2 parts by weight, relative to thequantity of carrier resin.

The aqueous dispersion according to the invention is preferentiallysuitable for a cataphoretic deposition of a coating on an electricallyconductive substrate which is connected as the cathode in anelectrocoating lacquering process. For carrying out the cathodicdeposition, the aqueous dispersion is diluted with water down to asolids content between 5 and 30%, preferably between 5 and 15%. The pHvalue is between 1 and 9, but preferably between 5 and 9. During thecathodic deposition, the dispersion is kept at temperatures between 15°and 40° C. The substrate to be coated is immersed in the dispersion andconnected as the cathode. The anode used is graphite or a noble metal. Adirect current is passed through the bath between the cathode and theanode. The deposition voltage is 20 to 500 volts. Under these conditionsa coating is deposited on the cathode. Deposition is carried out untilthe desired film thickness has been achieved.

It is a particular advantage that film thicknesses of 150 μm areobtained on the coated substrate even after a brief period. Depending onthe plastic powder chosen, periods as low as 10 seconds are adequate insome cases to obtain these film thicknesses. After the substrate hasbeen removed from the coating bath, the coating is rinsed with water andstoved for 15 to 60 minutes at temperatures between 140° C. and 220° C.In some cases it is advantageous to interpose a brief preliminary dryingstage at 100° C. before stoving.

It has been found, surprisingly, that the powder resin is deposited onthe cathode together with the carrier resin. This could not have beenexpected, since dispersions of finely divided powder resin cannot bedeposited by electrophoresis.

Since the electrocoating bath becomes depleted in both the carrier resinand the plastic powder during the deposition process, it is necessary toreplenish the bath with these substances, so that the originalcomposition of the aqueous dispersion is always maintained.

The properties of the stoved coating are excellent from a technologicalpoint of view. The corrosion resistance is surprisingly good and varieswith the nature of the solid powder lacquers. Using the aqueousdispersion according to the invention a very high film thickness isachieved, which, of course, somewhat impairs the throwing power. Thestoved film can be subjected without difficulty to further lacqueringusing conventional lacquers.

The Examples which follow are intended to illustrate the essence of theinvention, but not to limit it. Percentages relate to percentages byweight; parts relate to parts by weight.

EXAMPLE 1 (Preparation of a Component A)

960 g of an epoxide resin, which is based on bisphenol A and has anepoxide equivalent weight of 480, and 167 g of xylene are weighed into areaction flask, which is equipped with a stirrer, a thermometer, aninlet for nitrogen and a reflux condenser. This mixture is heated to100° C. and 570 g of a 70% strength solution of a diketimine from thereaction of diethylenetriamine with methyl isobutyl ketone in methylisobutyl ketone and 36.5 g of diethylamine are then added under anitrogen atmosphere. The mixture is stirred for 1 hour at 110° to 120°C. and then cooled. A cationic synthetic resin is produced whichcontains amino groups and has a solids content of 80%. Its viscosity,measured as a 50% strength solution in xylene, is 355 mPas.

EXAMPLE 2 (Preparation of a Component A)

960 g of an epoxide resin based on bisphenol A (epoxide equivalentweight of 480) and 220 g of xylene are weighed into a reaction flaskequipped with a stirrer, a thermometer, an inlet for nitrogen and areflux condenser. This mixture is heated to 100° C. and 146 g ofdiethylamine are added under a nitrogen atmosphere. The mixture isstirred for 1 hour at 110° C. to 120° C. and thereafter cooled. A resinsolution of 80% solids content is obtained. The viscosity, measured as a50% strength solution in butylglycol, is 35 mPas.

EXAMPLE 3 (Preparation of a Component B)

1,000 g of an epoxide resin, which is based on bisphenol A and has anepoxide equivalent weight of 850 (Epikote 1055), is melted at 130° C. ina reaction vessel equipped with a stirrer, a thermometer, an inlet fornitrogen, a reflux condenser and a feed funnel. 105 g of diethanolamineare added via the feed funnel at 120°-130° C. and allowed to react forone hour. 60 g of glacial acetic acid are then added, the reaction iscontinued for 30 minutes at 120°-130° C. and the batch is thendischarged. A solid product is obtained.

    ______________________________________                                        Melting point:   74° C.                                                Milliequivalents 0.359 milliequivalent/g                                      of acid:                                                                      Milliequivalents 0.489 milliequivalent/g                                      of base:                                                                      Viscosity:       910 mPas (50% strength                                                        solution in ethylglycol)                                     ______________________________________                                    

The solid product is pre-ground. A powder of a particle size of 20μ isthen prepared by fine grinding and sieving.

EXAMPLE 4 (Preparation of a Component B)

Under the reaction conditions of Example 3, 850 g of the epoxide resinin accordance with Example 3 is reacted with 149 g of a reaction productof 89 g of dimethylethanolamine with 60 g of acetic acid. A solidproduct is obtained.

    ______________________________________                                        Melting point:   103° C.                                               Milliequivalents 0.25 milliequivalent/g                                       of acid:                                                                      Milliequivalents 0.403 milliequivalent/g                                      of base:                                                                      Viscosity:       970 mPas                                                     ______________________________________                                    

A powder is prepared by using the procedure described in Example 3.

EXAMPLE 5 (Preparation of a Component B)

100 g of a powder of the resin composition in accordance with Example 4(particle size 500μ), 3 g of a leveling agent, 20 g of titanium dioxide(rutile grade), 8 g of aluminum silicate and 2 g of red iron oxide aremelted and kneaded together in a kneader in the manner customary for thepreparation of pulverulent coating agents. After solidifying, themixture is pre-ground and thereafter ground in a spiral jet mill to givea powder with a maximum particle size of 30μ and an average particlesize of 10-15μ.

EXAMPLE 6 (Preparation of a Component B)

1,500 g of ethyl acetate are weighed into a reaction flask in accordancewith Example 3 and heated, under a nitrogen atmosphere, to the refluxtemperature. A mixture of 300 g of styrene, 450 g of methylmethacrylate, 450 g of butyl acrylate, 300 g of glycidyl methacrylateand 30 g of azoisobutyrodinitrile is then added in the course of 3hours. The polymerization is continued for 2 hours under reflux afterthe end of the addition. 188 g of methylethanolamine are then added viaa tap funnel and reacted for 1 hour under reflux. 126.7 g of acetic acidare then added. After a further 30 minutes of reaction period at 70° C.,the ethyl acetate is distilled off in vacuo. The solid product (meltingpoint 84° C.) obtained is then processed into a fine-grained powder asin Example 3.

EXAMPLE 7

5.5 parts of glacial acetic acid are added to 100 parts of the carrierresin prepared according to Example 1. This mixture is dispersed withstirring in 520 parts of distilled water. 50 parts of the powderobtained in accordance with Example 3 are then stirred into thisdispersion. The dispersion thus prepared has a solids content of 20% anda pH value of 8.3. A phosphatized sheet of steel is immersed in thisdispersion and connected as the cathode. An immersed sheet of stainlesssteel is connected as the anode. A coating was deposited on the cathodesheet for 30 seconds on applying a direct current at a voltage of 200volts and a bath temperature of 20° C. The sheet which had been providedwith the coating was taken out, rinsed with fully demineralized waterand thereafter stoved for 20 minutes at 180° C. A hard, even, continuousfilm with an average film thickness of 36 μm is formed on the side ofthe sheet facing the anode.

EXAMPLE 8

35.7 parts of a 70% strength solution of a product from the reaction of174 g of toluylene diisocyanate, 130 g of 3-ethylhexanol and 44.7 g oftrimethylolpropane, 0.5 part of dibutyltin dilaurate and 5.5 parts ofglacial acetic acid are added to 100 parts of the carrier resin preparedin accordance with Example 1. This mixture is dispersed with stirring in706 parts of distilled water. 50 parts of the powder obtained inaccordance with Example 3 are stirred into this dispersion. Thedispersion obtained has a solids content of 18%. After an aging periodof 24 hours, it is deposited in accordance with Example 7. A hard, even,continuous film of 34 μm thickness is obtained.

The film is resistant towards methyl isobutyl ketone, and in thecorrosion resistance test as specified in German Industrial Standard DIN50,021 it was still in a satisfactory condition after 1,000 hours.

EXAMPLE 9

A coating agent is prepared in accordance with Example 8. Thephosphatized sheet of steel which is to be coated is bent at an angle of90° so that an L-shaped workpiece is produced which has vertical andhorizontal areas.

After depositing and stoving in accordance with Example 8, the filmthickness is measured. On the horizontal part it is 33 μm and on thevertical part it is 32 μm. The appearance of the coating was uniform andsmooth.

The trial is repeated after the bath has aged for 14 days. Filmthicknesses of 31 μm on the horizontal and 30 μm on the vertical areawere found.

This result shows that the compositions described have a very goodstability.

We claim:
 1. In a process for coating an electrically conductivesubstrate comprising connecting said substrate as the cathode in anelectrocoating process, immersing said connected substrate in anelectrocoating bath containing a cationic coating agent, carrying out anelectrodeposition of said cationic coating agent on said substrate toproduce said substrate coated with said cationic coating agent andhardening said substrate coated with said cationic coating agent, theimprovement comprising said cationic coating agent comprising an aqueousdispersion comprising a mixture of:A. water dispersed or water solublecationic synthetic resins containing basic groups prepared by reactingresins containing epoxide groups and bases selected from the groupconsisting of organic amines and Mannich bases followed by protonationwith acids to form a carrier resin; and B. dispersed, finely divided,ionic plastics selected from the group consisting of epoxide resins,polyester resins, acrylate resin, polyurethane resins and polyamideresins having ionic groups selected from the group consisting ofammonium, sulfonium and phosphonium ions neutralized with acids.
 2. Theprocess of claim 1, wherein component A is the reaction product ofpolyepoxide compounds having 2 to 3 epoxide groups per molecule withprimary or secondary amines and protonated with carboxylic acids andcomponent B has a particle size distribution in which at least 95% ofthe particles are smaller than 30 μm and said ionic plastics are secondsynthetic resins containing basic groups and the weight ratio of B to Ais 0.1 to 100/l.
 3. The process of claim 1, wherein component A is thereaction product of polyepoxide compounds having 2 to 3 epoxide groupsper molecule with Mannich bases and protonated with carboxylic acids andcomponent B has a particle size distribution in which at least 95% ofthe particles are smaller than 30 μm and the said ionic plastics aresecond synthetic resins containing basic groups and the weight ratio ofB to A is 0.1 to 100/l.
 4. The process of claim 1, wherein said aqueousdispersion has a solids content of 5 to 30%.
 5. The process of claim 1,wherein said hardening is carried out at a temperature between 140° C.and 220° C. by stoving and components A and B combine to form acompatible film having a thickness of about 150 μm.
 6. The process ofclaim 2, wherein components A and B have a glass transition temperatureof 30° to 150° C.
 7. The process of claim 3, wherein components A and Bhave a glass transition temperature of 30° to 150° C.
 8. The process ofclaim 1, wherein said aqueous dispersion contains pigments.
 9. Theprocess of claim 8, wherein said aqueous dispersion contains fillers.10. The process of claim 1, wherein said aqueous dispersion containsadditional cross-linking agents selected from the group consisting ofmelamine resins, blocked polyisocyanates and phenolic resins.